This invention relates to castable refractories, and in particular to castable refractories composed primarily of fused or vitreous silica.
The best quality refractories are usually furnished by the manufacturers as pressed and fired brick. Proper installation of brick refractories often requires the use of special shapes, cutting of the brick, and meticulous installation thereof by skilled masons. Many refractory users have resorted to the use of monolithic refractory linings, due to their lower initial cost and easier and quicker installation. These refractories, supplied as castables, plastics, ramming mixes, gunning mixes and trowelling mixes, are suitable for most applications. Their properties, however, are not equal to those of brick since bonding agents such as cements, clays or organic and inorganic chemicals are used which may have an adverse effect on the composite chemistry of the product, or may prevent maximum density from being achieved in forming the refractory lining.
Refractories made from pure silica provide excellent high temperature load bearing characteristics. Principal uses for silica refractories are in coke ovens, glass melting furnace crowns and, in Europe, as checker brick in blast furnace stoves. Although pure silica is a very refractory material, small additions of alkalies, alumina or lime will lower its refractoriness markedly. In the manufacture of silica brick, a level of 2.0 to 3.5% lime is desired so that sintering will occur on firing. However, lime contents exceeding this range, or alumina contents exceeding 0.5%, lower refractoriness to an undesirable level. For special applications of silica brick, the maximum alumina content allowed is 0.35%. A monolithic silica refractory having the same chemistry as silica brick must be bonded with portland cement, since the alumina content of calcium aluminate cement is too high. However, the amount of portland cement must be controlled and cannot exceed 2% for special applications of the silica brick and 3% for regular applications of silica brick, since portland cement contains a small amount of alumina and higher cement contents will cause the maximum alumina to be exceeded. At the relatively low levels of cement, additional lime is desired to allow adequate sintering of the brick when the brick is fired. Possible lime sources include hydrated lime, which is commonly used in the manufacture of silica brick, limestone, gypsum, plaster, calcium nitrate and whiting. As the foregoing are all calcium salts or hydrates, they loose weight on firing, causing a refractory containing them to loose density.
Further, most silica monolithic refractories are based on vitreous or fused silica grain which enables the refractories to be rapidly heated in service. At elevated temperatures, these refractories shrink to some degree as the grains sinter. This is objectionable in some applications, particularly in coke oven repairs, where dimensional stability on heating is a must.
Accordingly, it is an object of this invention to provide a silica castable which, in service, has the same properties as silica brick. Specifically, it is an object of this invention to provide a silica castable which does not loose weight on firing, nor does the castable shrink as the grains sinter.